If the popup is a real one, not a div in the page, the browser/system will provide the "X" to close the window, so you should only provide the one in the code, which should be as understandable as possible.
If you are generating the popup effect with a div, then you can provide the button yourself, as per your screenshot.
But, since your intention is to be sure that the user has read the text you presented to him, I'd say that you should try to avoid any way of closing the window that is not related to acknowledge the actual message, so, don't provide an "X".
We are used to interact with many kinds of confirmation boxes, popups and related interfaces, so when we scan the screen and we see a box with only one option, we know that it doesn't matter what we do, the only option is to close it, so the brain and the attitude is dismissive; many problems happen because of that, but that is another subject. So make the button something the user have to pay attention to, like "I have read and understood the text", or " I agree with ...", something that makes the user stop for a moment and really interact with your box or read the text if he hasn't done it yet.